1. Field of the Invention
The present invention relates to a catalytic converter for effectively cleaning the exhaust gas of an automotive internal combustion engine by removal of nitrogen oxide (NOx), carbon monoxide (CO) and hydrocarbons (HC).
2. Description of the Related Art
As is well known, the exhaust gas of an automotive internal combustion engine inevitably contains harmful substances such as NOx, CO and HC. In recent years, particularly, the restrictions on exhaust gas cleaning are increasingly strict for environmental protection.
A so-called three-way catalytic converter has been most widely used for removing the above-described harmful substances. The three-way catalytic converter utilizes, as an active substance, a precious metal or metals such as Pt, Pd and/or Rh for reducing NOx to N2 and for oxidizing CO and HC to CO2 and H2O. In this way, the three-way catalytic converter works as a catalyst both for oxidation and reduction.
Various researches have been made to improve the performance of a three-way catalytic converter. One of the three-way catalytic converters which have resulted from such researches utilizes cerium oxide (CeO2) which has an oxygen-storing capacity (OSC); that is, the capacity to occlude gaseous oxygen in the crystalline structure and to release the occluded oxygen from the crystalline structure. More specifically, CeO2 is added to a three-way catalytic converter for adjusting the oxygen concentration of gaseous atmosphere, so that excess oxygen in the gaseous atmosphere is occluded into the crystalline structure of CeO2 in an oxygen-rich state for assisting the catalytic converter in reducing NOx to N2 while releasing the occluded oxygen into the gaseous atmosphere in a CO— and/or HC-rich state for assisting the catalytic converter in oxidizing CO and HC to CO2 and H2O, respectively.
Meanwhile, there is an increasing demand for shifting the mounting location of the catalytic converter from below the body floor to the exhaust manifold which is near the engine, whereby the catalyst can be quickly warmed up after starting the engine. Due to such a location, however, the catalytic converter may be often exposed to high temperature of no less than 900° C. (or sometimes even higher than 1,000° C.). Thus, the catalytic converter needs to provide a high catalytic activity even at such a high temperature. Further, the catalytic converter is also required to provide a high catalytic activity at relatively low temperature before the engine is sufficiently warmed up upon start thereof.